RF devices for millimeter-wave (mm-wave) applications such as car radar may include an antenna integrated in the integrated circuit package, which may also be referred to as an antenna-in-package. At mm-wave frequencies, the RF-performance of the system is no longer determined only by the transceiver circuits and the antenna, but also strongly depends on the package and the interconnection between the transceiver and the antenna. An example package type used for antenna-in-package devices is a fan-out package for example the embedded wafer-level ball-grid-array (eWLB). The eWLB typically includes a mold compound surrounding the packaged semiconductor die or dice. The eWLB package may have one redistribution layer (RDL) on the same major surface of the package as the solder balls of the ball grid array (BGA). Some eWLB packages may have a second redistribution layer on the opposite major surface to the solder balls. Vias formed through the mold compound may be used to electrically connect the first and second redistribution layers. The opposite major surfaces of the eWLB package may also be referred to as the front-side and back-side of the package.
Antenna-in-packages using the eWLB package concept may form a planar antenna in one of the redistribution layers. To improve the power radiation of the antenna in the target direction, a reflector is typically implemented. In some implementations, this reflector uses the top layer metal of an external printed circuit board to which the package is soldered. In this case, the effectiveness of the reflector depends on the soldering process, since the height of solder balls defines the distance between the reflector and the antenna. A variation in this height can consequently affect the matching and insertion loss of the antenna.
An alternative example in eWLB packages with two redistribution layers integrates a reflector into the package using the metal layer of one of the redistribution layers and integrates an antenna into the other redistribution layer. In this case, the antenna performance strongly depends on the mold thickness since at higher frequencies the surface wave loss can be very high which can deteriorate the far-field radiation pattern and the antenna gain in the desired direction. This surface wave loss may result in the antenna efficiency being reduced to 40% or lower.
In general, the antenna performance is affected by the package thickness that, in the eWLB package, is electrically thick in the mm-wave frequency range. The eWLB package itself supports propagation of guided waves (surface waves) that bounce within the package making the antenna performance very sensitive to the package dimensions.
Furthermore, the antenna is in close proximity to the chip consisting mainly of material such as silicon or gallium-arsenide that has a high relative dielectric constant above 10. Consequently, a significant portion of the radiated power tends to couple with the integrated circuit die or chip rather than being radiated outside the package. This may result in asymmetric and degraded radiation patterns from the antenna.